This invention relates to a golf ball having excellent flight performance.
In general, golf balls are provided with a multiplicity of dimples of circular plane shape on their surface for the purpose of improving their aerodynamic properties. It is well known that the dimpled golf balls are far better in flight behavior than smooth golf balls free of dimples.
The flight distance of golf balls depends on the initial velocity, drag and lift acting on the ball in flight, spin rate, and other factors such as weather conditions. The initial velocity is largely governed by the constituent materials and structure of the ball. The dimples are correlated to the drag and lift in flight and the spin rate and thus affect the flight distance. A number of proposals have been made on the arrangement of dimples.
The dimple arrangement is generally determined by assuming the golf ball to be a regular polyhedron, typically a regular icosahedron or octahedron to divide the spherical surface into spherical polygons, using each spherical polygon as a unit, suitably arranging dimples of circular plane shape and of one type or plural types which are different in size, and expanding this arrangement over the entire spherical surface.
The dimple arrangement or distribution looks uniform when the overall spherical surface is viewed. However, when polygonal units, specifically triangular units of a regular icosahedron or octahedron are considered, the number of dimples on the delimiting line between adjacent triangular units is often smaller than in the remaining portion, that is, the lands (that is, the regions where no dimples are formed) are formed along the delimiting line over a relatively long distance. As a consequence, there is a tendency that the uniform arrangement of dimples is disrupted at the delimiting line. A golf ball with such a dimple arrangement often exhibits a difference in flight distance between pole hitting and seam hitting, lacking the symmetry of elevation angle and carry when hit. Provided that a ball is molded in a mold of two separable halves so that the ball has an equator plane corresponding to the parting plane of the mold, the term xe2x80x9cpole hittingxe2x80x9d means that the ball is hit to give a back spin about an axis passing the center of the equator plane parallel to the equator plane. The term xe2x80x9cseam hittingxe2x80x9d means that the ball is hit to give a back spin about an axis passing the opposed poles of the ball perpendicular to the equator plane.
As mentioned above, golf balls tend to exhibit flight differences depending on the hitting mode because of variances or biases in their manufacturing process and dimple arrangement. To compensate for such biases, various proposals have been made. Japanese Patent No. 2,569,515 discloses a golf ball which is formed to achieve the symmetry of elevation angle between the pole hitting and the seam hitting (to minimize the difference in elevation angle between the pole hitting and the seam hitting). The symmetry of carry is not always satisfactory in the high head speed region.
An object of the invention is to provide a golf ball which is improved in the symmetry of elevation angle and carry, and has good aerodynamic performance and an increased total flight distance.
According to the invention, there is provided a golf ball having a spherical surface, wherein when the spherical surface is divided into a plurality of substantially congruent spherical triangles, dimples are substantially equally distributed in the spherical triangles. The spherical triangles are minimum triangular units which are substantially congruent with each other. The dimples distributed in each minimum triangular unit include crossing dimples that each lie across at least one side of the minimum triangular unit. The total of the crossing lengths of the dimples which lie across the respective sides of the minimum triangular unit is 70 to 80%, and preferably 75 to 80%, of the total length of all the sides of the minimum triangular unit.
In one preferred embodiment, the minimum triangular unit is obtained by dividing the spherical surface into twenty spherical triangles of a regular icosahedron, and drawing perpendicular lines from the apexes of each said spherical triangle to divide the triangle into six triangular units. In another preferred embodiment, the minimum triangular unit is obtained by dividing the spherical surface at its equator into north and south hemispheres, and dividing each hemisphere about its pole along longitudes into six spherical triangular units having a vertex angle of 60xc2x0, thus defining twelve spherical triangular units in total. Also preferably, the dimples account for 70 to 80% of the entire surface area of the ball, that is, the dimple area coverage is 70 to 80%. As to the golf ball structure, a golf ball comprising a solid core of a single layer or plural layers and a cover of at least one layer enclosing the core is preferred.
The inventor has found that a golf ball having a dimple arrangement satisfying the above-mentioned specific requirement is improved in the symmetry of elevation angle and carry and will travel an increased total distance.